1. Field of the Invention
The present invention relates to a dispersion compensating optical fiber that compensates the chromatic dispersion and the dispersion slope of a dispersion-shifted optical fiber, an optical transmission line that includes the dispersion-shifted optical fiber and the dispersion compensating optical fiber, and a dispersion compensating module made of the dispersion compensating optical fiber.
2. Related Background Arts
In order to increase the transmission capacity in a Wavelength Division Multiplexing (WDM) transmission system, it is important to lessen the absolute magnitude of the cumulative chromatic dispersion of an optical transmission line as much as possible at a broad optical signal spectrum band. Generally, an attempt is made to reduce the absolute magnitude of the cumulative chromatic dispersion of an optical transmission line in a wide wavelength range by producing an optical transmission line by connecting plural kinds of optical fibers because it is difficult to do so in an optical transmission line using only one kind of optical fiber.
For example, Japanese Patent Application Laid-Open No. 6-11620 discloses a technique to reduce the absolute magnitude of the cumulative chromatic dispersion in the 1.55 xcexcm wavelength band by connecting a standard single-mode fiber (SMF) which has a zero dispersion wavelength in the vicinity of the 1310 nm wavelength and which has a chromatic dispersion of about 15 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 at the 1550 nm wavelength and a dispersion compensating fiber (DCF) which compensates the chromatic dispersion of the single mode fiber at the 1550 nm wavelength. It is set forth that in order to reduce the absolute magnitude of the cumulative chromatic dispersion of an optical transmission line in a broad wavelength range that includes the 1550 nm wavelength, the relationship (SDCF/DDCF)=(SSMF/DSMF) should be satisfied, when DSMF is the chromatic dispersion of a standard SMF at the 1550 nm wavelength, SSMF is the dispersion slope thereof, DDCF is the chromatic dispersion of a dispersion compensating optical fiber, and SDCF is the dispersion slope thereof.
When the chromatic dispersion at the 1550 nm wavelength of a non-zero dispersion shifted fiber (NZDSF) which has a positive small chromatic dispersion at the 1550 nm wavelength is represented as DDSF, and the dispersion slope thereof is represented as SDSF, SDSF/DDSF is substantially great as compared with SSMF/DSMF. Therefore, with a dispersion compensating optical fiber for SMF described in the specification of Japanese Patent Application Laid-Open No. 6-11620, it is impossible to compensate the chromatic dispersion of the dispersion-shifted fiber and reduce the absolute magnitude of the cumulative chromatic dispersion of an optical transmission line with respect to all of the wavelengths over a wide range that includes the 1550 nm wavelength.
Also, U.S. Pat. No. 5,838,867 discloses a technology for reducing the absolute magnitude of the cumulative chromatic dispersion at the 1.55 xcexcm wavelength band of an optical transmission line that is produced by connecting a non-zero dispersion shifted fiber and a dispersion compensating optical fiber which compensates the chromatic dispersion and the dispersion slope of the non-zero dispersion shifted fiber at the 1550 nm wavelength. However, in order to compensate both the chromatic dispersion and the dispersion slope of a non-zero dispersion shifted fiber, a long length of dispersion compensating optical fiber for NZDSF is needed because the dispersion compensating optical fiber for NZDSF of U.S. Pat. No. 5,838,867 has a small absolute magnitude of the chromatic dispersion.
For example, in the case of the non-zero dispersion shifted fiber described in Literature 1: S. Bigo, et al., xe2x80x9c1.5 Terabit/s WDM transmission of 150 channels at 10 Gbit/s over 4xc3x97100 km of TeraLight(trademark) fibrexe2x80x9d, ECOC ""99, PD (1999), the chromatic dispersion is +8 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 and the dispersion slope is +0.06 psxc2x7nmxe2x88x922xc2x7kmxe2x88x921 at the 1550 nm wavelength. In the case of the non-zero dispersion shifted fiber described in Literature 2: David W. Peckham, et al., xe2x80x9cReduced dispersion slope, non-zero dispersion fiberxe2x80x9d, ECOC ""98, pp.139-140 (1998), the chromatic dispersion is +4 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 and the dispersion slope is +0.046 psxc2x7nmxe2x88x922xc2x7kmxe2x88x921 at the 1550 nm wavelength. In the case of the non-zero dispersion shifted fiber described in Literature 3: Valeria L. da Silva, et al., xe2x80x9cError free WDM transmission of 8xc3x9710 Gbit/s over km of LEAF(trademark) Optical Fiberxe2x80x9d, ECOC ""97, No.448, pp.154-158 (1997), the zero dispersion wavelength is 1506 nm to 1514 nm, and the chromatic dispersion is about +4 to 5 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 and the dispersion slope is about +0.1 psxc2x7nmxe2x88x922xc2x7kmxe2x88x921 at the 1550 nm wavelength. To compensate the chromatic dispersion of 80 km of non-zero dispersion shifted fibers described in these literatures requires 8 km to 16 km lengths of the dispersion compensating optical fiber for NZDSF described in U.S. Pat. No. 5,838,867. Furthermore, in such case, it is impossible to sufficiently compensate both the chromatic dispersion and the dispersion slope at the same time.
Generally, a dispersion compensating optical fiber for a dispersion-shifted optical fiber is prone to leakage of the fundamental mode light at a slight bend thereof, and the bending loss of the fundamental mode light is large. Therefore, the transmission loss increases when it is formed into a cable and installed, or when it is wound on a bobbin to form a dispersion compensating module. As a result, in an optical transmission system that performs optical communication by allowing light signals to propagate through an optical transmission line that is produced by connecting a dispersion-shifted optical fiber and a dispersion compensating optical fiber for the dispersion-shifted optical fiber, the transmission loss is large in the optical transmission line. Accordingly, increasing the span of a transmission section (i.e. a distance between repeater stations) is not feasible, and it is difficult to achieve further increase in capacity of optical communication.
An object of the present invention is to provide a dispersion compensating optical fiber that can, with a relatively short length thereof, compensate both the chromatic dispersion and the dispersion slope of a non-zero dispersion shifted fiber at a wide spectrum band that includes the 1550 nm wavelength. Another object of the present invention is to provide an optical transmission line that includes a non-zero dispersion shifted fiber and a dispersion compensating optical fiber whose transmission loss is relatively small. A further object of the present invention is to provide a dispersion compensating module that compensates both the chromatic dispersion and the dispersion slope of a non-zero dispersion shifted fiber.
In order to achieve these objects, an optical fiber that satisfies the following relational expressions is provided:
xe2x88x92250 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921xe2x89xa6DDCFxe2x89xa6xe2x88x9240 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921
0.015 nmxe2x88x921xe2x89xa6SDCF/DDCFxe2x89xa60.030 nmxe2x88x921,
where DDCF is the chromatic dispersion at the 1550 nm wavelength and SDCF is the dispersion slope thereof.
In this optical fiber, the effective area may be in a range of from 13 xcexcm2 to 17 xcexcm2, from 17 xcexcm2 to 20 xcexcm2, or equal to or more than 20 xcexcm2 at the 1550 nm wavelength. The cutoff wavelength may be from 1.2 xcexcm to 1.8 xcexcm, and the transmission loss may be 0.5 dB/km or less at the 1550 nm wavelength. The term xe2x80x9ccutoff wavelengthxe2x80x9d as used herein means the cutoff wavelength of the LP11 mode that is measured in a state where an optical fiber of 2 m length is loosely wound once at a radius of 140 mm.
This optical fiber may have a central core region having a first refractive index, a first cladding region surrounding the central core region and having a second refractive index that is smaller than the first refractive index, a second cladding region surrounding the first cladding region and having a third refractive index that is greater than the second refractive index, and a third cladding region surrounding the second cladding region and having a fourth refractive index that is smaller than the third refractive index. In this case, the variation of the ratio (SDCF/DDCF) may be equal to or less than 10% when the outer diameter of the central core region is altered by 2% while the ratios of the outer diameters between the respective regions (the central core region, first cladding region, second cladding region and third cladding region) are kept constant. The region of the outermost layer may be silica glass doped with F element, P element or Cl element.
Moreover, an optical transmission line is provided which is installed at a transmission section by connecting the above-mentioned optical fiber and a dispersion-shifted optical fiber whose chromatic dispersion is in the range of +2 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 to +10 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 at the 1550 nm wavelength and whose dispersion slope is in the range of +0.04 psxc2x7nmxe2x88x922xc2x7kmxe2x88x921 to +0.12 psxc2x7nmxe2x88x922xc2x7kmxe2x88x921at the 1550 nm wavelength. This optical transmission line may have a multiplexer/demultiplexer that introduces pump light for Raman amplification, and the deviation of the average chromatic dispersion may be equal to or less than 0.2 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 at a wavelength ranging from 1535 nm to 1560 nm.
In addition, a dispersion compensating module containing an optical fiber in a coil form is provided. Such coil may have an arbor or no arbor in it. In the latter case, the coil may be fixed by filling the inner space of the module with a resin.
The above and further objects and novel features of the invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.